Process for polishing aluminum



Patented June 2, 1953 UNI TED PATENT OFFICE PROCESSEORPOLIS H-ING ALUMINUM Frederick Harold Hesch, Spokane, Wash, as-

's'ignorto- Kaliser Aluminum"& hemical Corpo ration, a'corporation of Delaware No Drawing. Application June 27,1949, .SerialNo. 101,693

7 Claims.

"This invention relates .to the brightening of aluminum and aluminum alloys. Moreparticw larly, the invention relates to .a composition and method for chemical brightening of aluminum and its alloys.

Although aluminum is ordinarily considered a bright metal, it often presents a dull or mattelike finish dueto the oxide films: formed on its surfaces during processing. Many aluminum and aluminum alloy products :are fabricated from mill finished'sheet having such a characteristic dull appearance, which leads to a demand by fabricators for means to impart a bright, lustrons finish to .their products. lone-widely used method of polishing aluminum and its alloys is by mechanical 'bufiingwith a suitable abrasive. However, this mechanical polishing is expensive and is not easily adaptable to articles having intricate shapes and inaccessible surfaces. Other methods known as electropolishing and elect-rm brightening involve subjecting the aluminum article to, an electrolytic treatment and producing desirable lustrous finishes. 'I-Iowever, these methods are both slow and expensive.

It is therefore, a primary object and purpose of the invention to provide a com-position and method for the chemical brightening or polishing of aluminum. A further object is to provide a chemical solution or bright dlD-CQlllDOEltlOlfl "for aluminum and aluminum alloys which is char acterized by its'economy in use, short immersion time required to obtain the desiredeflect, and an ease'of application. 'Another object is to produce of a'lum-inum and aluminum :alloy articles polished or brightened surfaces by means of a chemical bright dip composition and process which effectively removes the (killing oxide-films from themetallic surfaces and imparts thereto a high luster.

Chemical brightening of aluminum has pro-- viously been accomplished using a combination of nitric and hydrofluoric acids. The hydrofiuoric acid readily dissolves any oxide film and attacks the metal. The nitric acid, which is essentially inactive toward aluminum, "serves: to dissolveany metallic smut which might adeposit out of solution. onto themetal'suriace being treated. However, the etching action of the proved difficultto'control regardless of the concentration or temperature employed, and uniform results could not'be consistently obtained.

It has been discovered-according tothe-present invention that if a noble metal, such as-gold, palladium or platinum, is used as an electrode and is electrically connected to theialuminum, or alloy article immersed in the solution of nitric and hydrofluoric acids, a superior brightening of the "surfaces of the article is obtained. The term noble metal as used in the specification and claims is intended to include gold, platinum and palladium.

Platinum, palladium and-gold being at the extreme end of the electromotive force series-ore impervious to attack'by the acidic constituents of the bath. *In addition, they formwith aluminum, a galvanic cell or couple having a high electricpotential. It is believed that this fact accounts for the greatly enhanced brightening effect. 'It is-only necessary'in forming the oouple that these noble metal electrodes "and =the aluminum or aluminum alloy be electricallyconnected either within the cell or externally. "Without this couple, the solution *forms an etched o'r frosted finish; but when the noble metal'eleotrode is connected to the aluminum :and'the couple is formed, a bright "finish is 'obtained. Gold and platinum are the preferredelectrode materials.

The aluminum article is "connected or joined to the noble metal, 'which latter may be in the form of strips or of 'a' bath container-lining or inner jacket. lhe electrode and the work inlay be electrically connected, for example, outside the cell bymeansof a wire, or'with-in theoell by direct contact. "The electrode area is pref ably approximately that of the area of the piece although good results a're obtained where e electrode area is not directly related to the area of the work piece.

While the combination of'nitric acid, hydrofiuoric acid and metallic gold or platinumprm duce bright surfaces, it was foundthat addition of. a mild inhibitor such as glycerine would simprove the brightness. Any of'the lower nrolocu lar weight polyhydric aliphatic alcohols. such-as ethylene glycol, diethylene glycol, propylene gly-- col, mannitoland sorbitol are also suitable as inhibitors.

..'I'he. amounts of nitric and hydrofluoric acids, and polyhydric alcohol used are preferably within the following ranges:

Thefiuoride ion is preferably introduced] in the form of hydrofluoric acid, although any'suit,

presence of nitric acid may be used, for example,

ammonium fluoride.

The fluoride ion concentration should be maintained at a value corresponding to that produced by amounts of hydrofluoric acid within the abovestated range.

The proper hydrogen ion concentraion is primarily maintained by the use of nitric acid in an amount within the above indicated range.

Thus, the bright dip composition of the present invention comprises an aqueous acid solution containing hydrogen, nitrate, and fluoride ions in concentration corresponding to that produced by asolution containing from about .5 to about 6% nitric acid, and from about .01 to about 0.5% hydrofluoric acid by weight for use in conjunction with noble metal electrodes electrically connected to the aluminum or aluminum alloy work piece. In addition, the solution preferably contains from about .5 to about 5% by weight of a lower molecular weight polyhydric aliphatic alcohol.

Excellent brightening results are obtained when the hydrogen and fluoride ion concentrations are maintained at any combination of values corresponding to that produced by a solution containing nitric acid and hydrofluoric acid, and preferably a polyhydric alcohol inhibitor, in any combination of constituent amounts within the ranges set forth above, or any combination of equivalent substances which will produce a solution of substantially the same chemical composition.

It is to be noted that a satisfactory degree of brightening is obtained when using any of the constituents in slightly smaller or larger amounts than indicated above. However, the desired effect is more readily obtained by operating within the stated ranges.

The chemical bath may be operated at temperatures of from about 100 F. to the boiling point. The desired brightening is accomplished with a shorter immersion time at the higher temperatures of from about 190 F. to boiling, and accordingly, this range is preferred. The time of immersion of the articles being treated in the bath should be 'sufficient to produce the desired brightenin yet insuflicient to cause any undue etching of the metal. It is, of course, dependent primarily on solution temperatures and concentrations and may be widely varied. An immersion time of from about one-half to about ten minutes is most desirable.

The following example of a chemical dip solution producing maximum brightness is illustrative of a specific embodiment, and is not to be construed as a limitation of the invention:

Percent Volume Coneentra- Constituent tion cue/liter lggiglltigogf BN so 10% HNOs) 2(48% HF) .1 Glycerol 25 .l Water-Balancc for the required time and thereafter thoroughly water rinse and dry the treated article. However, it is advantageous that the metal to be processed be cleaned or degreased by the use of a mild, inhibited alkaline cleaner. This is preferably accomplished by immersion in a heated solution of the cleaner for a few minutes, for example, two minutes in solution at 180-200 F., followed by a thorough rinsing prior to introduction into the bright dip solution.

Agitation of the bath i not essential, but is recommended since it produces more uniform results and tends to decrease the required immersion time. Mechanical or air agitation may be used, but the former is preferable since it does not cool the solution as does air agitation.

The process may be operated on a continuous or semicontinuous basis, or it may be conducted as a batch process. In case of the former, the components are gradually depleted and calculated additions are periodically added to maintain proper concentrations.

The chemical polishing treatment may be conducted in any suitable tank or other apparatus provided with a lining impervious to the corrosive action of the nitric and hydrofluoric acids at the operating temperatures. A particularly useful lining is Karbate-a treated carbon product resistant to attack by all chemicals except those which are highly oxidizing. Karbate is manufactured in molded slabs, blocks and other shapes.

The heating of the solution may be accomplished by the use of Karbate tubes or heat exchangers, or, if electrical heating is desired Karbate shielded immersion heaters may be employed.

The results obtained with the chemical bright dip of the present invention do not produce a surface smoothing effect in comparison to mechanical buffing or electropolishing. However, the surface luster or brightness compares very favorably with that produced by electrobrightening, while the chemical bright dip requires only a short immersion time and no electric current. In short, the improved chemical bath produces the desired result much more economically.

What is claimed is:

1. A process for the chemical brightening of aluminum and aluminum alloys which comprises electrically connecting the work to a noble metal electrode, and subjecting the work to the action of an aqueous acid solution consisting essentially of hydrogen, nitrate and fluoride ions in concentration corresponding to that produced by a solution containing from about .5 to 6% nitric acid. and about .01 to 0.5% hydrofluoric acid by weight, and maintaining the solution at a temperature of from about F. to the boiling point.

2. A process for the chemical brightening of aluminum and aluminum alloys which comprises forming a galvanic couple between the work and a noble metal electrode in an aqueous acid solution consisting essentially of from about .5 to 6% nitric acid and about .01 to 0.5% hydrofluoric acid by weight, and maintaining the solution at a temperature of from about 100 F. to the boiling point for a time suflicient to brighten the work.

3. A process according to claim 1 in which the solution contains from about .5 to 5% of a lower molecular weight polyhydric aliphatic alcohol as an inhibitor.

4. A process according to claim 2 in which the solution contains from about .5 to 5% of a lower molecular weight polyhydric aliphatic alcohol as an inhibitor.

5. A process according to claim 2 in which the noble metal electrode is gold.

6. A process according to claim 2 in which the noble metal electrode is platinum.

7. A process for the chemical brightening of aluminum and aluminum alloys which comprises treating the metal with an acid aqueous solution consisting essentially of from about .5 to 6% nitric acid, about .01 to 0.5% hydrofluoric acid, and about .5 to 5% glycerol by weight of solution in the presence of a noble metal electrode, said electrode being electrically connected to the metal work, maintaining the solution at a temperature of from about 190 F. to the boiling point, withdrawing and rinsing the work after an immersion of from about one-half to about ten minutes.

FREDERICK HAROLD HESCH.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR THE CHEMICAL BRIGHTENING OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES ELECTRICALLY CONNECTING THE WORK TO A NOBLE METAL ELECTRODE, AND SUBJECTING THE WORK TO THE ACTION OF AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF HYDROGEN, NITRATE AND FLUORIDE IONS IN CONCENTRATION CORRESPONDING TO THAT PRODUCED BY A SOLUTION CONTAINING FROM ABOUT .5 TO 6% NITRIC ACID, AND ABOUT .01 TO 0.5% HYDROFLUORIC ACID BY WEIGHT, AND MAINTAINING THE SOLUTION AT A TEMPERATURE OF FROM ABOUT 100* F. TO THE BOILING POINT. 